bi-

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In an old, common method used to indicate the presence of an acidic hydrogen, sodium hydrogen sulfate is called "sodium bisulfate" and sodium hydrogen carbonate is called "sodium bicarbonate". This method is not recommended by IUPAC and does not denote a “doubling up” of a specific group, which is reserved for the Greek prefix di-, as in carbon dioxide ‎(“CO2”).

This prefix often corresponds to an English possessive ’s appended to the preceding word. For example, Diné bizaad means literally “the People their-language”, equivalent to “the People’s language” (i.e., Navajo language).